1. can see no reason not to do something
so I don't see why not
The phrase 'I don't see why not' is used in spoken English and means 'I can't find a reason not too'. If you say 'I don't see why not' you are giving your approval or permission for something to go-ahead. This phrase is used most often between parents and children. A common response to children asking for permission to do something is: "I don't see why not."